US President Barack Obama urged nations on Wednesday (Mar 23) to unite against terrorism after the deadly attacks in Brussels and said wiping out the Islamic State extremist group was his "top priority."

The Islamic State claimed responsibility after bombers killed 31 people and wounded 270 at Brussels airport and a metro station on Tuesday, leaving European and world leaders once more grappling for ways to tackle the militant threat.

"Groups like ISIL can't destroy us. They can't produce anything. They're not an existential threat to us. They are vicious killers and murderers," Obama said during a visit to Argentina.

"The United States will continue to offer any assistance that we can to help investigate these attacks and bring attackers to justice. We will also continue to go after ISIL aggressively until it is removed from Syria and removed from Iraq and is finally destroyed," he said.

"The world has to be united against terrorism and we can and we will defeat those who threaten the safety and security not only of our own people but of people all around the world. So that is the top priority of ours."

Obama spoke alongside his Argentine counterpart Mauricio Macri at the start of the US president's stop in Argentina, where he arrived following a landmark visit to Cuba.

The two sides in Buenos Aires signed agreements to boost trade and also cooperate on counter-terrorism, peacekeeping and health threats such as the Zika virus that has struck Latin America.

WORKING TO DESTROY ISBelgian prosecutors said two brothers with links to November's major attacks in Paris were among the suicide bombers who struck Brussels.

Prosecutors identified Ibrahim El Bakraoui as one of two men who blew themselves up in the Zaventem airport departure hall while his brother Khalid struck at the Maalbeek metro station in the attacks on the symbolic heart of Europe.

Police stepped up a manhunt for a third airport assailant whose bomb failed to go off.

Ahead of Obama's meeting with Macri, a man claiming to have a bomb threatened to blow up a building six blocks from the presidency in Buenos Aires, witnesses told media. Police arrested the man, aged in his fifties, and evacuated the building housing a state radio station. No one was reported hurt.

Obama said US airstrikes on IS targets in Syria and operations against their bases in Iraq were "working" to weaken the extremist group. "We are going to continue to press on them until we have driven them out of their strongholds and until they're destroyed," Obama said.​"While we are doing that, we're also extraordinarily vigilant about preventing attacks in the homeland and working with our allies to prevent attacks in places like Europe, but as I said before, this is difficult work."